The present invention relates to a shift control system for an automatic transmission capable of effecting an automatic shift mode, in which a shift is executed on the basis of a running state of a vehicle, and a manual shift mode in which a gear stage is selected by the manipulation of a driver.
An automatic transmission for a vehicle basically effects a drive condition suitable a running state such as a throttle opening or a vehicle speed by selecting a gear stage on the basis of the running state. In recent years, there has been frequently used an automatic transmission, in which the running state is detected and converted into electric signals by various sensors, in which the electric signals are computed to determine a gear stage, and in which signals based upon the computation results are outputted to predetermined solenoid valves so that the shift may be executed by actuating the solenoid valves. The so-called "electronic control type automatic transmission" executes the shift directly on the basis of the output signals coming from the computer for the aforementioned computations but can effect the shift by outputting a shift signal on the basis of other input signals without resorting to the computation results based on the running condition. There is known an automatic transmission which is enabled to accomplish the manual shift by making effective use of the degree of freedom of such electronic control type automatic transmission. This known automatic transmission is constructed such that a shift lever is manipulated to output a manual shift signal so that an arithmetic unit may decide a gear stage to be set, on the basis of the manual shift signal, and may output a shift signal for achieving the gear stage.
A shift unit for changing a running range in the automatic shift mode and for switching the shift mode to the manual one and selecting a gear stage in the manual shift mode is disclosed in U.S. Pat. Nos. 4,905,530, 4,987,792, 5,009,128, 5,044,220 and 5,062,314.
In the shift unit disclosed in those U.S. Patent specifications, the individual range positions for the parking (P) range, reverse (R) range, neutral (N) range and drive (D) range in the automatic shift mode are set along a straight groove for guiding a shift lever. In parallel with this straight groove, there is formed a short groove which has one end portion assigned to an upshift position and its other end portion assigned to a downshift position. And, those two grooves are so connected as to transfer the shift lever.
In the shift unit described above, the shift mode is switched from the automatic to manual ones by moving the shift lever from the D-range position to the aforementioned short groove. If the shift lever is moved in the short groove to the upshift position, an upshift switch disposed in the upshift position outputs an ON signal so that an upshift to a gear stage higher by one stage is executed. On the other hand, if the shift lever is moved to the downshift position, a downshift switch disposed in the downshift position outputs an ON signal so that a downshift to a gear stage lower by one stage is executed.
Thus, the shift unit described above can effect the upshift or downshift step by step but cannot perform the so-called "skip shift", in which the gear change is skipped to another higher or lower by two stages or more. In order to effect a shift substantially approximating the skip shift, the shift lever has to be promptly manipulated two times or more so that the shift unit has an unsatisfactory operability.
In U.S. Pat. No. 5,056,376 and Japanese Patent Laid-Open No. 157855/1986, on the other hand, there is disclosed a shift unit which can select the individual gear stages directly in the manual shift mode. In this shift unit, a guide plate for regulating the position of the shift lever is formed with one transverse groove and five or six longitudinal grooves branched from the transverse groove to the two sides. Of these longitudinal grooves, one groove formed at the end portion of the transverse groove is assigned to the parking (P) range position, the reverse (R) range position, the neutral (N) range position and the drive (D) range position, and the remaining four or five grooves are Individually assigned to the shift positions, i.e., the positions for the first to fourth or fifth speeds in the manual shift mode. Moreover, the automatic shift mode is set by setting the shift lever in the D-range position so that the shift is effected on the basis of the detected running state such as the vehicle speed and the throttle opening. Moreover, if the shift lever is positioned in one of the longitudinal grooves assigned to the first to fourth or fifth speeds, the shift mode is switched to the manual one to set the gear stage assigned to that particular longitudinal groove.
According to this shift unit, the gear stage can be directly selected in the manual shift mode. Despite a this advantage, however, the individual range positions in the automatic shift mode and the individual gear stage positions in the manual shift mode are provided in the completely different areas so that a wider space is required for the individual range positions and shift positions. In short, the size of the shift unit is enlarged. Moreover, the stroke for manipulating the shift lever is elongated to deteriorate the operability.
On the other hand, in the shift unit provided with the individual gear stage positions independently in the manual shift mode, there is disposed in each gear stage position a switch which is turned ON so that the shift may be effected on the basis of the signal outputted from the switch. The shift control cannot be effected unless it is decided by an electronic unit in terms of the OFF operations of all the switches that the drive (D) range for the automatic shift is selected. If any switch experiences a short-circuit failure, the D-range cannot be achieved so that the vehicle cannot run in the automatic shift mode. Moreover, if the manual shift switch corresponding to a lower-speed gear stage is short-circuited during the run in the D-range, the electronic control unit decides that the manual shift is executed to the gear stage corresponding to that broken switch, and causes a shift down suddenly. In the manual shift mode, on the other hand, a control can be effected to prefer the higher-speed gear stage by processing the switch signal so as to prevent a downshift to a lower-speed gear stage due to the short-circuit of the switch corresponding to the lower-speed gear stage during the run at the higher-speed gear stage. Then, the shift may probably be fixed at the higher-speed gear stage to make the start impossible if the switch at the higher-speed gear stage is short-circuited.
In the system of the prior art described above, moreover, the selection of the individual ranges in the automatic shift mode and the selection of the individual gear stages in the manual shift mode can be accomplished by moving the shift lever along the transverse groove and by moving the shift lever along the longitudinal grooves perpendicular to the transverse groove. However, the moving direction of the shift lever from the transverse groove is perpendicular to the transverse grove commonly in case a range is to be selected in the automatic shift mode and in case a gear stage is to be selected in the manual shift mode, but the selections are different in the branching positions from the transverse groove in the automatic shift mode and in the manual shift mode. Therefore, in order that the driver may set the shift lever in the gear stage position or the range position desired by himself, he is required to move the shift lever to the desired longitudinal groove after he has confirmed that the shift lever was moved to the end portion of the transverse groove. Thus, the system of the prior art requires the confirming operation of the driver so that its operability is not satisfactory.
Moreover, the shift unit of the prior art is advantageous in that the gear stage can be directly selected in the manual shift mode, but its operation can be carried out by manipulating the shift lever only unlike the manual transmission. Thus, a shifting operation may be performed to a lower-speed stage during the run at a high speed. As a result, the overrun of the engine may probably arise.
Incidentally, in the manual shift mode, the engine braking is effected at all the gear stages. Depending upon the construction of the gear train, therefore, at a shifting time in the manual shift mode, it is necessary to effect the so-called "engagement change", in which engine braking frictional engage means for a gear stage before the shift is disengaged while engine braking frictional engage means for a gear stage after the shift is engaged. If, in this case, the engagement of the other frictional engage means is premature with respect to the disengagement of one frictional engage means, a shifting shock is caused by the tic-up. On the other hand, if the engagement of the other frictional engage means is delayed, the shifting response is deteriorated by the slip. The solution of this problem is of great importance because it is essential especially in the manual shift mode to enhance the shifting response better than in the automatic shift mode.